Separation of pyrrole



Patented June 6, 1944 UNITED STATES PATENT oFrics ard Clark, Sprin fieldTownship,

Montgomery County, Pa and John Wesley Waldron, Muncie,

Ind., ass'ignors portion, New York, York to Allied Chemical & Dye Cor-N. Yi, a corporation of New No Drawing. Application May 10,1943,

Serial No. 486,456

10 Claims. (Cl. $560-$190) This invention relates to the recovery of pyr-. role from mixtures of heterocyclic nitrogen compounds in which itoccurs. M

Pyrrole occurs in admixture with picoline and other pyridine homologs ina number Ofnatural sources of heterocyclic nitrogen compounds. Forexample, a mixture of heterocyclicnitrogen compounds recovered fromcertain coal tars contains a substantial proportion .of. pyrrole. Theseheterocyclic nitrogen compounds, includ.. ing pyrrole, pyridine, and thevarious pyridine homologs such as picolines and lutidines, arecustomarily extracted from coke-oven distillates and other sources ofthese heterocyclic nitrogen compounds by means of an aqueous solution ofmineral acid, usually sulfuric acid. After liberation of the nitrogencompounds from their acid solutions by means of alkali, the mixtures ofnitrogen compounds are Compounds such as pyridine, boiling point 116 C.,can generally be obtained in reasonably pure form by fractionaldistillation, but many .of the remaining compounds cannot be separatedby ordinary distillation methods alone. Instead, the fractions obtained,even when they are .of relatively narrow boiling range, containincreasing numbers of the various heterocyclic nitrogen compounds ineach fraction.

Although pyrrole alone boils at about 129.8 -C.,

in admixture with heterocyclic nitrogen compounds such as picolines itforms azeotropes from which the pyrrole cannot be separated by ordinaryfractional distillation. These azeotropes are of the high'boiling type,boiling generally within the range of about 149-14S C. For example amixture of 20% pyrrole and 80% of a 3- and i-picoline mixture iii-about;equal proportions was found to distil through the temperature range of14688-1410 C., whereas the 3- and i-picoline mixture alone distilled at144;? to 144.5" C. and the pyrrole alone distilled at 129.8 to 129.95"C. Upon fractional distillation of a mixture of heterocyclic nitrogencompounds recovered from certain coal tars containing pyrrole, fractionstaken Within the range 140- 148 C., even closeout fractions boiling forexample within about 1 of 147 C., have been found to contain, insubstantial proportions, 3.-picoline (beta-picoline) boiling point 143.80., 4-picoline (gamma-picoline), boiling point 144.8 C., generally asmall amount of 2,6-lutidine, boiling point 143.8 C., and, in addition,a substantial quantity of pyrrole, boiling point 129.80. Mixtures ofthis fractionally distilled;

kind have found limited practical application, mainly as specialsolvents, and are of relatively low economic value; in fact, thepresence of'even a few percent pyrrole limits the usefulness ofpicoline-lutidine mixtures for many solvent purposes because thepresence'lof pyrrole makes the mixture waterinsoluble. However, thepyrrole, which is thus detrimental in a picoline-lutidinemixture, isitself a rare chemical of considerablevalue. For example, pyrrolederivatives are of importance aspharmaceuticals, agricultural chemicals,e. g., insecticides, perfume bases," and 'antiseptics; however the highcost ofpreparing pyrrole by the complicated procedures now in use limitsits employment in these and other fields.

Since the componentsmaking up the heterocyclic nitrogen compoundfractions containing pyrrole form constantboiling azeotropes, fractionaldistillation is obviously unsatisfactory as a means for resolving themixtures. Methods heretofore proposed in the published prior art forseparating pyrrole from such mixtures have been generally unsatisfactoryin products, yields, and production costs.- One method proposed for suchseparation, for example, is to treat the heterocyclic nitrogen compoundmixture with an excess of; sulfuric acid to hold the picolines and otherpyridine homologs in combination and subject the resulting mixture toprolonged steam, distillationto remove the pyrrole which is only looselyheld in the sulfuric acid-pyridine homolog mixture. This method has beenfound unsatisfactory, however, since the heating of pyrrole inthe-presence of sulfuric acid causes resinificationof theygreater partofthe pyrrole to polymers insoluble in hydrocarbon oils, nitrogen basesor water, forming a jelly-like precipitate which it is impractical toseparate. This method is. thus ineffective for separation of pyrrolefrom pyridine homologs and, moreover,

all or most of the valuable pyrrole is converted into a product oflittle or no utility. Methods for separation of pyrrole have also beendisclosed in the published prior art involving forming potassiumpyrroles. However, the .yields and production costs of these methodshave been found so unfavorable that the methods have not been consideredfeasible for industrial use.

It is an object of this invention to provide a process for separatingsubstantially pure pyrrole from admixture with other heterocycliccompounds notreadily separable therefrom by distillation; 1

It is afurtherobject of this invention to provide a commerciallyattractive method for recovering pyrrole from fractions of heterocyclicnitrogen compounds, particularly of coke-oven origin, boiling in therange from 140- 148 C., which fractions contain 3-picolineQ4-picoline,and pyrrole in substantial proportions, and may contain 2,6-lutidine.

We have discovered that pyrrole may be ef'--'-- fectively andeconomically separated from mixtion to separate asdistillate a productenriched-- in pyrrole. The distillation residue may then be treated, e.g., with aqueous alkali, to liberate a heterocyclic nitrogen compoundproduct low in pyrrole, i. e., enriched in the pyridine homologs thataccompanied the pyrrole in the original mixture.

In the process of our invention, pure pyrrole and a pure pyridinehomolog product may further be produced from the above products, 1. e.,the pyrrole-enriched material. recovered as distillate, and thepyridine-homolog-enriched material recovered from the distillationresidue. by fractionally distilling the pyrrole-enriched material toseparate, as distillate, substantially pure pyrrole and by fractionallydistilling arate, as distillate, a substantially pure pyri-- thepyridine-homolog-enrichedmaterial to sepdine homolog product. In each ofthese two fractional distillations, the distillation residue consists ofa constant-boiling pyrrole-pyridine homolog azeotrope of thehigh-boiling type, having a composition that is generally within therelatively narrow range of to pyrrole and 75% to 85% pyridine homologs,by weight; the exact composition of the high-boiling azeotrope willdepend, at least in part, on the relative amounts of 3-picoline,4-picoline and 2,6-lutidine in the pyridine homolog component of themixture. The high-boiling pyrrole-pyridine homolog azeotrope remainingas residue from each'of the two distillations is added to a new batchof. pyrrole-containing heterocyclic nitrogen compound mixture to betreated by the process of our invention, while the pure pyrrole and purepyridine homolog material are withdrawn as prod ucts.

Our invention further comprises a process,

in which the above-described separation of pyrrole is combined withsteps for resolving into its components the above pyridine homologmaterial obtained as coproduct with the py role. This pyridine homologmaterial generally contains 3-picoline, 4-picoline, and 2,6-lutidine.Thus our invention is'applicable to resolving into its components'amixture of pyrrole, 3-picoline, 4-pic0line, and 2,6-lutidine.

The process of our invention may be advan-,

tageously applied to the recovery of pyrrole from heterocyclic nitrogencompound mixtures obtained from coke-oven distillates. crackedpetroleum, shale tar, bone oils, the products of denitrogen compoundscontains components of widely varying boiling points, the mixture isfirst subjected to fractional distillation whereby a fraction boiling inthe range 140-148 C. containing predominantly pyrrole and othernitrogen-compounds not separable therefrom by ordinary distillation isobtained. Our invention comprises a process in which such afractionation is combined with the above-described separation ofpyrrole. I

As above stated, a relatively strong acid should be employed in thepartial neutralization. A strong mineral acid such as sulfuric acid orhydrochloric acidis preferably employed. However, any relatively strongacid, i. e., an acid hava ing a minimum dissociation constant of 10-preferably no lower than 10- may be employed, provided such acid doesnot react deleteriously with the pyridine homologs or the pyrrole underthe conditions of the process. Examples of such acids other thansulfuric and hydrochloric acids, include phosphoric acid, oxalic acidand benzenesulfonic acid.

In a preferred method of carrying out the process of our invention, apyrrole fraction, as above described, suitably a fraction boiling in therange -148 0. containing pyrrole, 3-,

picoline, 4-picoline, and small amounts of 2,6- lutidine, is mixed witha limited amount of sulfuric acid or hydrochloric acid, substantiallyless than the stoichiometric quantity of acid required for reaction withthe nitrogen bases to.

form nitrogen base sulfates or hydrochlorides.

As above indicated, the heterocyclic nitrogen compound mixture is thuspartially neutralized to the point where it reacts slightly alkaline tomethyl orange, i. e., to the point where it has a pH preferably in therangev of about 4.4 to 5.0. Whenthe amount of acid is thus carefullycontrolled, there will be no excess acid in the mixture and,accordingly, the pyrrole upon heating 'will not resinify as it does whenheated in the presence of a strong acid, such as sulfuric acid orhydrochloric acid.

The mixture slightly alkaline to methyl orange, thus obtained, issubjected to steam distillation either at atmospheric pressure or underreduced pressure. When a dilute acid, e. g., 30% sulfuric acid, has beenemployed for the partial neutralizationstep, the steam distillation maybe accomplished merely by heating the mixture to boil off water and apyrrole-enriched nitrogen compound mixture. ;Steam distillation iscontinued until the distillateruns practically free of oil. The aqueousdistillate is then made alkaline, e. g., by addition of caustic soda, toreduce the solubility of pyridine homologs present in the pyrrole-richproduct in minor amount. The oil which separates out is decanted.

The distillationv residue now low in pyrrole and consisting mainly ofpyridine homolog sulfates or hydrochlorides is treated withalkali toliberate the'fnitrogen compounds whichjaredecanted. Two products arethus recovered from tillatioma pyrrole-rich product and h'o'mologproduct lowinpyrrole.

As indicated above, pyrrole tends to form azeotropes of the high-boilingtype with pyridine homologs such as the picolines. The constantboilingmixture thus formed with a 3-picoline-4- picoline-2,6-lutidine mixture,as above stated, has a composition generally ranging from 15% to 25%pyrrole and 75 to 85% pyridine homologs by weight. Accordingly, both thepyrrolerich product and the pyridine-homolog-rich product obtained abovemay be distilled to leave this high-boiling azeotropic mixture as stillresidue, in the first case taking off substantially pure pyrrole asdistillate and in the second case taking off substantially pure pyridinehomologs as distillate. The distillation residue consisting of thepyrrole-pyridine-homolog azeotropic mixture recovered from eachdistillation may be added to pyrrole-containing heterocyclic nitrogencompound mixture entering the process, while the pure pyrrole and purepyridine homologs are withdrawn as products.

The pyridine homolog mixture thus recovered, generally containing3-picoline, 4-picoline, and 2,6-lutidine, may be resolved into itscomponents by methods described in copending applications, e. g. themethods of copending. applications Serial No. 456,257, filed August 26,1942, serial No. 441,557, filed May 2, 1942, and serial No. 452,369,filed July 25, 1942. V

The following examples are illustrative of the process of our invention:

Example 1 798 pounds of an industrial beta-gamma picoline fraction(containing 16% pyrrole and 84% of a mixture of 3-picoline, 4-picoline,and 2,6-lutidine) were charged to a 500-gallon glass-lined kettle usedas a still, along with 987 pounds of 27.6% aqueous sulfuric acidsolution; the mixture had a pH of about 5. The kettle was equipped witha small condenser and a 175-gallon receiver with appropriate lines torun the system under vacuum. Distillation was carried out for 8 hoursunder 26inches of vacuum. The distillate was separated into two layers:796 pounds of water containing 0.64% nitrogen compounds and 246 poundsof crude pyrrole containing 8.5% water.

The'distillation residue was neutralized with 782 pounds of 38% causticsoda mixed with 497 pounds of water. A pyridine homolog layer, low inpyrrole, was decanted. This amounted to 553 pounds and contained 26.5%water.

The pyrrole product and the pyridine homolog product were fractionallydistilled. From the crude pyrrole, a pure pyrrole product amounting to35% ing a high-boiling pyrrole-pyridine-homolog azeotrope as stillresidue. From the crude pyridine homolog material a pure pyridinehomolog product amounting to 85% of the still charge was taken asdistillate, leaving as still residue a high-boilingpyrrole-pyridine-homolog azeotrope. The high-boilingpyrrole-pyridine-homolog aze otrope remaining as still residue in eachcase was set aside for further treatment with the next batch ofindustrial beta-gamma picoline.

Example 2 Another sample of the same beta-gamma picoline fraction usedin Example 1 was charged to a still with an amount of 30% sulfuric acidsolution sufficient to neutralize the mixture to a point just on thealkaline side of the methyl orange end point (pH about as described inExample 1. In this case, steam distillation was homologmaterialjliberated from the steam of the charge was taken as distillate,leavcarried out at atmospheric pressure for-.shours. Crude pyrrole'wasrecovered from the distillate and crude pyridine homolog material fromthe still residue, as described in Example 1.- 2

In this case the crude pyrrole contained a somewhat higher contentofpyridine homologs owing to some dissociation of neutral pyridine homologsulfates upon distillation at atmoss pheric pressure. Fractionaldistillation of the crude pyrrole produced about-14% .of pure'pyrrole asdistillate, the still residue being the highe boiling. pyrrolepyridinehomolog azeotrope. Fractional distillation of the crude pyridinedistillation residue produced about of. a pure pyridine homolog productas distillate, the .still residue being the high-boilingpyrrole-pyridine homolog azeotrope. t.

Ercmple 3 material, pure pyridine homolog product amount;

ing to 78% of the still charge was taken as 'distillate'. Example 4 Asampleof the same beta-gamma ,picol ine fraction used in Examples 1, 2,and. 3 was treat-v ed by the same general procedure outlined in Ex-,ample 3, ,except that the steam distillation was carriedout under avacuum of-about 26 inches mercury, thus holding the steam temperaturearound 60 C.

Upon fractional distillation of the crude pyrrole, purepyrroleamountingto 10.7% of the still charge was taken as distillate, Uponfractional distillation of the crude pyridine homolog material, a purepyridine homolog product amounting to 577% of the still charge was takenas distillate.

Since certain changes'may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in alimit' ing sense;

l in aprocessfor separating. pyrrole from a mixture thereof withpyridine homologs not read-.' ily separable therefromby distillation,the steps that comprise adding to the mixture an acid that 'does notaffect the nitrogen compounds deleteriously, said acid having adissociation constant not lower than about l0 in amount substantiallyless than the stoichiometric amount for reaction with the nitrogen basespresent, and subjecting the resulting mixture to steam distillation toseparate as distillate a nitrogen compound product enriched in pyrrole.

2. In a process for separating pyrrole from a mixture thereof withpyridine homologs not readily separable therefrom by distillation, thesteps that comprise substantially neutralizing to a point just on thethe-gmixture with'an acid. that (does not affect the'nitrogen' compoundsdeleteriously, said acid having. a dissociation. constant not lower thanabout to apoint. where the .mixture has a pH'.not lower than about.4.4,and-subjecting the resulting mixture to steam distillation to sepa-vrate, as distillate a nitrogen compound product em'iched in pyrrole.

:3. In a process for separating.pyrrolefroma mixture thereofwithpyridine homologs not readilytseparable therefrom by distillation,the steps thatcomprisesubstantially neutralizingthe mixture with astrong mineral acid that does not af-.

fect the nitrogen compounds deletc'riously to a point where the mixture.has a pH not lower than about 4.4, and subjecting the resulting mixtureto steam distillation to separateas distillate a nitrogen compoundproduct enriched in pyrrole.

4. In a process for separating pyrrole from a mixture thereof with.pyridine homologs not readily separable therefrom by distillation, thesteps that comprise substantially neutralizing the mixture with an acidthat does not affect the V nitrogen compounds deleteriously, said acidhaving a dissociation constant not lower than about 10%, to a-pointwhere the mixture has a pH not lower than about 4.4, subjecting theresulting mixture to steam distillation to s eparate as'distillate anitrogen compound product enriched in' riched in pyrrole, treatingthesteam distillation residue to liberate a nitrogen compound productenriched inpyridine homologs, and subject'- ing ach of these nitrogencompound products to fractional distillation to separate as distillatestherefrom substantially pure pyrrole and'a substantially pure'pyridinehomolog product respectively.

6.:In a process for recovering pyrrole from a mixture thereof withpyridine homologs not readily separable therefrom by distillation, thesteps that-comprise substantially neutralizing said mixture with astrongmineral acid that does not affect the nitrogen compoundsdeleteriously to apoint where the resulting mixture tests slightlyalkaline to methyl orange indicator, subjecting the resulting mixture tosteam distillation to separate as distillate: a nitrogen compoundproduct enriched in-pyrrole, and subjecting this nitrogen compoundproduct, to fractional distillation to separate as distillate therefromsubstantially purepyrrole. V

' '7. In a process for recovering pyrrole from a mixture thereof withpyridine homologs not readily separable therefrom by distillation, the

steps that comprise substantially neutralizing said mixture withsulfuric acid to a point where the resulting mixture has a pH in therange of about 4.4 to 5, subjecting the resulting mixture to steam.distillation to separate as distillate a nitrogencompound productenriched in pyrrole, treating-the steam distillation residue with alkalito liberate a nitrogen compound product enriched in pyridinehomologs,,andsubjecting each of these nitrogen compound products tofractional distillation to separateas distillates therefromsubstantially, pure, pyrrolezand a substantially pure pyridinehomologproduct respectively.

8. In a process for recovering pyrrole from a mixture thereof with.pyridine homologs not readilyseparable therefrom by distillation, thesteps that comprise substantially neutralizing said mixture withsulfuricacid to a point where the resulting mixture has a pH in therange of about 4.4 to 5, subjecting. the resulting mixture to steamdistillation under reduced pressure to separate as distillate a nitrogencompound product enriched in pyrrole, and subjecting thisnitrogen'compoundproduct to fractional distillation to separate asdistillate -therefrom substantially pure pyrrole.

9. In a process for recovering pyrrole from a heterocyclic nitrogencompound mixture con taining, besides pyrrole, pyridine homologs notreadily separable therefrom by distillation, the steps that comprisefractionating said mixture to obtain a pyrrole distillate cut boiling inthe range -148 C., substantially neutralizing saidcut with sulfuric acidto a point where the resulting mixture has .apH in the range of about4.4 to 5, subjecting the resulting mixture to steam distillation to 1separate as distillate a nitrogen compound product enriched in pyrrole,treating the steam distillation residue with alkali to liberate anitrogen compound product enriched in pyridine homologs, and subjectingeach of these nitrogen compound products to fractional distillation toseparate as distillates therefrom substantially pure pyrrole andsubstantially pure pyridine homolog material respectively.

10. In a process forrecovering pyrrole from a mixture thereof withpyridine homologs not readily separable therefrom by distillation, thesteps that comprise substantially neutralizing the mixture with an acidthat does not affect the nitrogen compounds deleteriously, said acidhaving a dissociation constant not lower than 10 to a point where themixture tests slightly alkaline to methyl orange indicator, subjectingthe resulting mixture to steam distillation to separate as distillate anitrogen compound product enriched inpyrrole, treating the steamdistillation residue to liberate a nitrogen compound product enriched inpyridine homologs, subjecting the product enriched in pyridine homologsto fractional distillation to separate as'distillate asubstantially-purepyridine homolog product and resolving this pyridinehomolog product into its individual component pyridine homologs.

PERCY JULIUS COLE.

CHARLES RICHARD CLARK.

JOHN WESLEY WALDRON.

